arXiv:math-ph/0604005v1 3 Apr 2006
Dequantization of Noncommutative Spaces and Dynamical Noncommutative Geometry Freddy Van Oystaeyen Department of Mathematics & Computer Science University of Antwerp Middelheimcampus Middelheimlaan 1, 2020 Antwerpen, Belgium e-mail :
[email protected] February 4, 2008 Abstract The purely mathematical root of the dequantization constructions is the quest for a sheafification needed for presheaves on a noncommutative space. The moment space is constructed as a commutative space, approximating the noncommutative space appearing as a dynamical space, via a stringwise construction. The main result phrased is purely mathematical, i.e. the noncommutative stalks of some sheaf on the noncommutative space can be identified to stalks of some sheaf associated to it on the commutative geometry (topology) of the moment space. This may be seen as a (partial) inverse to the deformation–quantization idea, but in fact with a much more precise behaviour of stalks of sheaves. The method, based on minimal axiomatics necessary to rephrase continuity principles in terms of partial order (noncommutative topology) exclusively, leads to the appearance of objects like strings and (M–)branes. Also, spectral families and observables may be defined and studied as separated filtrations on the noncommutative topology! We highlight the relation with pseudo– places and generalized valuation theory. Finally, we hint at a new notion of “space” as a dynamical system of noncommutative topologies with the same commutative shadow (4 dimensional space–time if you wish) and variable (but isomorphic) moment spaces, which for special choices may be thought of as a higher dimensional brane–space.
Introduction Noncommutative spaces obtained by quantization-deformation may to some extent be traced back to the base space they have been deformed from. A noncommutative topology in the sense of [VO1] has a so-called commutative shadow
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that turns out to be a lattice. However, the lack of geometric points in a noncommutative space contrasts the set-theory based commutative geometry with its function theory and corresponding analysis. Looking for a converse construction for quantization, a better commutative approximation of a noncommutative space is needed; this construction should allow tracing of physical aspects like observables, spectral families etc... Different notions of noncommutative space have been introduced e.g. noncommutative manifolds [Co], noncommutative “quantized” algebras [ATV], [VdB], general quantization-deformations [Ko]. In these theories the actual geometric object is left as a virtual object and results deal usually with noncommutaive algebras or categories thought of as rings of functions or categories of modules. In the author’s approach, started in [VO1], a noncommutative topology is at hand and then sheaf theory should to some extent replace function theory. Applied to Physics this model seems to allow some explanation of quantum phenomena but for explicite calculations and geometric reasoning a commutative model is unavoidable because at some point coordinatization and the use of real or complex numbers becomes necessary. In the model we propose these commutative techniques may be carried out in the dequantized space. The construction of this commutative approximation of noncommutative space derives from a dynamical version of noncommutative geometry that may also be seen as a noncommutaive topological blow-up and blow-down construction. Restricting assumptions to bare necessities one finds that all structural properties only depend on a few intuitive axioms at the level of ordered (partial) structures. Respecting this philosophy we started from a totally ordered set (not even necessarily a group), T and suitably connected noncommutative spaces Λt , t ∈ T . A stringwise construction then yields a new spectral space, called the moment space, at each t ∈ T , with its topology in the classical sense i.e. a commutative space. Sheaves on lattices were already termed quantum sheaves, but now we view (pre-)sheaves on noncommutative topologies fitting together in a kind of dynamical geometry and connect them to sheaves on the moment spaces. The idea that commutative moment spaces are good approximations of noncommutative spaces is reflected in the main result : the stalks at points of the moment space at t ∈ T equal stalks at a point at some Λt′ , t′ ∈ T for some t′ in a prescribed closed T -interval containing t. The philosophy here is that there may not exist enough geometric points in the noncommutative geometry (Λt ) at t ∈ T but it holds in the moment space at t because it encodes dynamical information in some T -interval containing t. The construction is abstract and rather general, but when looking for applications in Physics one is only interested in one case ... reality ? Then one would think of T as a multidimensional irreversible “time” with “dimension” big enough to make up for the missing geometruc points in the noncommutative space Λt at t ∈ T , i.e. the T -dimension explains the difference in dimension between the noncommutative space, or the moment space, and that of its space of points (the commutative shadow). Whereas the commutative shadow represents the abstract space where the mathematics takes place, e.g. space-time, the moment space represents the space where the mathematics 2
of observed and measured items takes place, and because observing or measuring takes time the latter space has to encode dynamical data. Stated as a slogan : “Observation Creates Space from Time”. Moreover, when observing a smallest possible entity, we would like to think of this as an observed point i.e. a point in the moment space, at t say. Tracing this via the dynamical noncommutative geometry to the commutative shadow (thinking of this as the fixed space we calculate in) the observed point appears as a string (it may be open or closed). On the other hand the point in the moment space is via a string of temporal points (not geometric points) in the noncommutative space also traceable to a possibly “higher dimensional string” in the moment space, in fact a tube-like string connecting opens in (the topologies of the noncommutative spaces blown down to) the spectral topology of the moment space (also identified to one commutative space for example). Of course this suggests that a version of M -theory already appears in the mathematical formalism explaining the transfer between commutative shadow and moment space. Moreover the incertainty principle also marks the difference between the two commutative spaces, in some sense expressing a quantum-commutativity of the noncommutative spaces, for example phrased in sheaf theoretical terms. We have aimed at a rather self contained presentation of basic facts and definitions of the new mathematical objects in particular ... generalized Stone spaces and noncommutative topology. The methods of this paper can be adapted to other types of noncommutative spaces e.g.quantales,... . Further generalization, e.g. to noncommutative Grothendieck topologies, has not been included, this would be an exercise using some material from [VO1]. We include some comments about specific examples and further mathematical elaborations using deformations of affine spaces or other nice classical (algebraic) varieties is left as work in progress.
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Preliminaries on Noncommutative Topology
We consider (partially ordered) Λ with 0 and 1. When Λ is equipped with operations ∧ and ∨, we say that Λ is a noncommutative topology if the following axioms hold : i) for x, y ∈ Λ, x ∧ y ≤ x and x ∧ y ≤ y and x ∧ 1 = 1 ∧ x = x, x ∧ 0 = 0 ∧ x = 0, moreover x ∧ . . . ∧ x = 0 if and only if x = 0. ii) For x, y, z ∈ Λ, x ∧ y ∧ z = (x ∧ y) ∧ z = x ∧ (y ∧ z), and if x ≤ y then z ∧ x ≤ z ∧ y, x ∧ z ≤ y ∧ z. iii) Properties similar to i. and ii. with respect to ∨, in particular x ∨ x ∨ . . . ∨ x = 1 if and only if x = 1. iv) Let id∧ (Λ) = {λ ∈ Λ, λ ∧ λ = λ}; for x ∈ id∧ (Λ) and x ≤ z we have : x ∨ (x ∧ z) ≤ (x ∨ x) ∧ z, x ∨ (z ∧ x) ≤ (x ∨ z) ∧ x.
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v) For x ∈ Λ and λ1 , . . . , λn ∈ Λ such that 1 = λ1 ∨ . . . ∨ λn , we have x = (x ∧ λ1 ) ∨ . . . ∨ (x ∧ λn ) = (λ1 ∧ x) ∨ . . . ∨ (λn ∧ x). There are left (right) versions of this definition as introduced in [VO.1]., but we do not go into this here. In fact we restrict attention to the situation where ∨ is a commutative operation and Λ is ∨-complete i.e. for an arbitrary family F of elements in Λ, ∨F exists in Λ, where ∨F is characterized by the property : λ ≤ ∨F for all λ ∈ F and if λ ≤ µ for all λ ∈ F then ∨F ≤ µ. In case only v) is dropped we call Λ a skew topology; this is sometimes interesting e.g. the lattice L(H) of a Hilbert space H is a skew topology and condition v) does not hold ! Here L(H) is the lattice of closed linear subspaces of H with respect to intersection and closure of the sum. The restriction to abelian ∨ (most interesting examples are like this) entails that id∧ (Λ) is closed under the operation ∨. Moreover on id∧ (Λ) we introduce a new operation ∧. defined by σ∧. τ = ∨{γ ∈ id∧ (Λ), γ ≤ σ ∧ τ } for σ, τ ∈ id∧ (Λ). Let us write SL(Λ) for the set id∧ (Λ) with the operations ∧. and ∨; then SL(Λ) is easily checked to be again a skew topology (with ∨ commutative and being ∨-complete).
1.1
Lemma (cf. [VO.1] or [VO.2] 2.2.3. and 2.2.5)
If Λ is a skew topology ∨-complete with respect to a commutative operation ∨, then SL(Λ) is a lattice satisfying the modular inequality. A subset X ⊂ Λ is directed if for every x, y ∈ X there is a z ∈ X such that z ≤ x, z ≤ y; we say that X is a filter in Λ if it is directed and for x ∈ X, x ≤ y yields y ∈ X. Two directed sets A and B in Λ are equivalent if they define the same filter A = B where for any directed set A we put A = {λ ∈ Λ, there is an a ∈ A such that a ≤ λ}. Let D(Λ) be the set of directed subsets in Λ and we write A ∼ B of the directed subsets A and B are equivalent, we write [A] for the equivalence class of A and let C(Λ) be the set of classes of directed subsets of Λ. We introduce a partial order in C(Λ) by putting A ≤ B if B ⊂ A and [A] ≤ [B] induced by the partial order on D(Λ) defined by the foregoing. For A ˙ = {a ∧ b, a ∈ A, b ∈ B}, A∨B ˙ = {a ∨ b, a ∈ A, b ∈ B} and B in D(Λ) define A∧B ˙ ˙ and : [A] ∧ [B] = [A∧B], [A] ∨ [B] = [A∨B].
1.2
Lemma
If Λ is a skew topology, resp. a noncommutative topology, then so is C(Λ) with respect to the partial order and operations ∧ and ∨ as defined above. The canonical map Λ → C(Λ), λ 7→ [{λ}] is a map of skew topologies. We simplify notations by writing [{λ}] = [λ] and call such an element classical in C(Λ). A directed set A in Λ is idempotently directed if for every a ∈ A there exists an a′ ∈ A ∩ id∧ (Λ) such that a′ ≤ a; in this case [A] ∈ i∧ (C(Λ)) but these elements of C(Λ) may be thought of as obtained from a directed set having a cofinal subset of “commutative” opens (the idempotents belonging to the commutative shadow S((Λ)). We write Id∧ (C(Λ)) for the subset of id∧ (C(Λ))
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consisting of the classes of idempotently directed subsets of Λ; the elements [A] of Id∧ (Id∧ (C(Λ)) are called strongly idempotents. We identify Λ and the image Q of Λ → C(Λ), then observe that Id∧ (C(Λ)) ∩ Λ = id∧ (Λ). We shall write (Λ) for the skew (noncommutative) topology obtained by talking ∧-finite bracketed expression P (∧, ∨, xi ) in terms of strong idempotents xi ∈ Id∧ (C(Λ)); similarly we write T (Λ) for the skew (noncommutative) topology obtained by taking ∧-finite bracketed expressions p(∧,Q∨, λi ) in idempotents λi ∈ id∧ (Λ). Q It is not hard to verify : (Λ) = (T (Λ)). Moreover C(T (Λ)) satifies the same axions (i. to v.) as Λ and Q T (Λ) but with respect to Id∧ (C(Λ)). The “strong” commutative shadowQof is obtained by restricting ∧. on id∧ (C(Λ)) to Id∧ (C(Λ)) and viewing SLs ( ) as the lattice structure induced on Id∧ (C(Λ)).
1.3
Lemma
If Λ isQa ∨-complete noncommutative topology such that ∨ is commutative then : SLs ( ) = C(SL(Λ)).
1.4
Definition. Generalized Stone Topology
Consider a skew topology Λ and C(Λ). For λ ∈ Λ, let Oλ ⊂ C(Λ) be given by Oλ = {[A], λ ∈ A}. It is very easy to verify : Oλ∧µ ⊂ Oλ ∩ Oµ , Oλ∨µ ⊃ Oλ ∪ Oµ , hence the Oλ define a basis for a topology on C(Λ) termed : generalized Stone topology. This definition obtains a more classical meaning when related to suitable point-spectra constructed in C(Λ). We say that [A] in C(Λ) is a minimal point of Λ if A is a maximal filter, i.e. A 6= Λ but if A $ B where B is a filter then B = Λ; it follows that a minimal point is necessarily in id∧ (C(Λ)) and it is indeed a minimal nonzero element of the poset C(Λ). An irreducible point [A] of Λ is characterized by either one of the followingequivalent properties : i) [A] ≤ ∨{[Aα ], α ∈ A} yields [A] ≤ [Aα ] for some α ∈ A. ii) If ∨{λα , α ∈ A} ∈ A then λα ∈ A for some α ∈ A. More general types of points may be considered, e.g.the elements of a so-called quantum-basis, cf [VO.2], but we need not go into this here. Under some suitable condition often (but not always) present in examples the irreducible points in Id∧ (C(Λ)) are exactly those that are ∨-irreducible in C(Λ) (e.g. if Λ satisfies the weak FDI property, cf. [VO.1], Proposition 5.9.). Define the (irreducible) point-spectrum by putting : Sp(Λ) = {[p], [p] an (irreducible) point of Λ}. Put p(λ) = {[p] ∈ Sp(Λ), [p] ≤ [λ]} for λ ∈ Λ, then p(λ ∧ µ) ⊂ p(λ) ∩ p(µ), p(λ ∨ µ) = p(λ) ∪ p(µ). Thus the p(λ) define a basis for a topology on Sp(λ) called the point-topology. Write SP (Λ) for Sp(Λ) ∩ Id∧ (C(Λ)) and refer to this as the Point-spectrum (capital P ). For λ ∈ Λ we consider P (λ) = {[P ] ∈ SP(Λ), [P ] ≤ [λ]} and this indices the Pointtopology on SP(Λ); this time we even have P (λ ∧ µ) = P (λ) ∩ P (µ) and this time we even have P (λ ∧ µ) = P (λ) ∩ P (µ) and this defines a topology on 5
SP(SL(C(Λ))). Similar constructions may be applied to the minimal pointspectrum QSp(Λ) on Sp(Λ) the generalized Stone topology is nothing Q but the point-topology. In the foregoing Λ may be replaced by T (Λ) or (Λ) with topologies induced by the generalized Stone topology on the point spectra always again being called : generalized Stone topology. Finally the generalized Stone topology can also be defined on the commutative shadow SL(Λ), which is a modular lattice, and then the topology induced on QSP(SL(Λ)) is exactly the Stone topology of the Stone space of SL(Λ). In the special case Λ = L(H) (only a skew topology) the generalized Stone space defined on QSP(L(H)) is exactly the classical Stone space as used in Gelfand duality theory for L(H). Warning : L(H) does not satisfy the axiom v. and whereas QSP(L(H)) is rather big, Sp(L(H)) = SP(L(H)) is empty ! Moreover over L(H) there are no sheaves but there will be many sheaves over C(L(H)) making sheaffication of a separated presheaf over L(H) possible over QSP(L(H)). The foregoing fixes a context for results in the sequel, however the methods in Sections 2-5 are rather generic and can be applied to other notions of noncommutative spaces.
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Dynamical Noncommutative Topology
The realization of a relation between the static and the dynamic goes far back, at least to D’Alembert but the notion that the first may be used to study the latter may be not completely well founded. Almost existentialistic problems defy the correctness of a mathematical description of so-called physical reality. So we choose for a converse approach constructing space as a dynamical noncommutative topological space and defining geometrical objects as existing over some parameter-intervals. Noncommutative continuity is introduced via the variation of an external parameter in a totally ordered set T (if one wants to consider this as a kind of time, fine... but then this time is an index, not a dimension). This is philosophically satisfying because momentary observations which are only abstractly possible (real measurement takes time) put us in the discrete-versus-continuous situation of noncommutative geometry ! Let T be a totally ordered set and for every t ∈ T we give a noncommutative space Λt . This can have several meanings, in the sequel we take this to mean that Λt is the generalized Stone space constructed on C(Xt ) for some skew topology Xt as in Section 1. This is just to fix ideas, in fact one could just as well restrict to topologies induced by the genaralized Stone topology on point spectra of any type, see also Section 1 or [VO1], or take pattern topologies as introduced in [VO1,2], or go to other theories and take quantales etc... For t ≤ t′ in T we have ϕtt′ : Λt → Λt′ which are poset maps respecting ∧ and ∨; when t = t′ we take ϕtt = 1Λt to be the identity of Λt and when t ≤ t′ ≤ t′′ then we demand that ϕt′ t′′ ◦ ϕtt′ = ϕtt′′ , where notation for composition of maps is conventional i.e. writing the one acting last frst. If At ⊂ Λt is a directed set then ϕtt′ (At ) ⊂ Λt′ , for t ≤ t′ , is again a directed set. 6
It is easily verified that if we start from a system {Xt , Ψtt′ , T } defined as above, we obtain a similar system {C(Xt ), Ψett′ , T } where Ψett′ : C(Xt ) → C(Xt′ ) is given by putting : Ψett′ ([A]) = [Ψtt′ (A)], for [A] ∈ C(Xt ) and t ∈ t′ in T . In case it is interesting to view Λt as coming from some Xt via C(Xt ) we may restrict attention to systems given by ϕtt′ , t ≤ t′ in T , stemming from ψtt′ on Xt by extension as indicated above. Note that not every system {C(Xt ), ϕtt′ , T } has to derive from a system {Xt , ψtt′ , T } in general.
2.1
Lemma
Any system of poset maps ϕtt′ , t ≤ t′ in T , defines a system of poset maps ϕett′ , t ≤ t′ in T . If the maps ϕtt′ respect the operations ∧ and ∨ in the Λt then so does ϕett′ for C(Λt ), t ∈ T . In this situation ϕtt′ maps ∧-idempotent elements of Λt to ∧-idempotent elements of Λt′ (also ∨-idempotent to ∨-idempotent) moreover if [At ] is strongly idempotent in C(Λt ) then [ϕtt′ (At )] is a strongly idempotent element of C(Λt′ ), for every t ≤ t′ in T , Proof First statements follow obviously from : for directed sets A and B, ˙ ˙ ϕett′ ([A] ∧ [B]) = ϕett′ ([A∧B]) = [ϕtt′ (A∧B)] ˙ tt′ (B)] = [ϕtt′ (A)] ∧ [ϕtt′ (B)] = [ϕtt′ (A)∧ϕ ˙ In case λ ∈ Λt is idempotent for t ≤ t′ in T . Similar with respect to ∨, using ∨. in Λt then ϕtt′ (λ) ∧ ϕtt′ (λ) = ϕtt′ (λ ∧ λ) = ϕtt′ (λ for t ≤ t′ in T . Finally if A is idempotently directed look at ϕtt′ (a) for a ∈ At ; by assumption there exists some µ ∈ id∧ (Λt ) such that µ ≤ a, thus ϕtt′ (µ) ≤ ϕtt′ (a) and ϕtt′ (µ) ∈ id∧ (ϕtt′ (At )), for t ≤ t′ in T . Consequently ϕtt′ (At ) is idempotently directed too. Q The skew topology t , introduced after Lemma 1.2. is called the pattern topology of Xt , i.e. it is obtained by taking all ∧-finite bracketed expressions with respect to ∨ and ∧ in the letters of Id∧ (C(Λt )).
2.2
Corollary
Q Q The system {Λt , ϕtt′ T } induces a system { t , ϕtt′ | t , T }, satisfying the same properties, on the pattern topologies. In general the ϕtt′ , t ≤ t′ , do not map poits of Λt to points of Λt′ , t ≤ t′ , neither does ϕtt′ respect the operation ∧. of the commutative shadow SL(Λt ), i.e. the ϕtt′ do not necessarily induce a system on the commutatve shadows !
2.3
Axioms for Dynamical Noncommutative Topology (DNT)
A system {Λt , ϕtt′ , T } is called a DNT if the following five conditions are satisfied :
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DNT.1 Writing 0, resp. 1, for the minimal, resp. maximal element of Λt (we shall assume these exist throughout) then ϕtt′ (0) = 0, ϕtt′ (1) = 1 for all t ≤ t′ in T . DNT.2 For all t ∈ T, ϕtt = 1Λt and for t ≤ t′ ≤ t′′ in T , ϕt′ t′′ ◦ ϕtt′ = ϕtt′′ . Moreover, all ϕtt′ preserve ∧ and ∨. Hence DNT.1. and 2. just restate that {Λt , ϕtt′ , T } is as before. DNT.3 If for some t ∈ T , 0 < x < y in Λt , then there is a t < t1 in T such that for z1 ∈ Λt1 satisfying ϕtt1 (x) < z1 < ϕtt1 (y) there is a z ∈ Λt , x < z < y, such that ϕtt1 (z) = z1 . DNT.4 For every t ∈ T and 0 < x < z < y in Λt there exist t1 , t2 ∈ T such that t1 < t < t2 and for every t′ ∈]t1 , t2 [ we have either t ≤ t′ and ϕtt′ (x) < ϕtt′ (z) < ϕtt′ (y), or t′ ≤ t and then if x′ < y ′ in Λt′ exist such that ϕt′ t (x′ ) = x, ϕt′ t (y ′ ) = y then there also exist z ′ in Λt′ such that x′ < z ′ < y ′ and ϕt′ t (z ′ ) = z. Taking the special case y = 1 and y ′ = 1 then we see that a nontrivial strict relation in Λt is alive in an open T -interval containing t. DNT.5 T -local unambiguity. In the situation of DNT.3, resp. DNT.4, the t1 ∈ T , resp. t1 and t2 , may be chosen such that z ∈ Λt is the unique element such that ϕtt1 (z) = z1 , resp. x′ , y ′ , z ′ in Λt , are the unique elements such that ϕt′ t (z ′ ) = z, ϕt′ t (y ′ ) = y, ϕt′ t (x′ ) = x, or when t ≤ t′ the x, y, z, are unique elements mapping to ϕtt′ (z), ϕtt′ (y), ytt′ (x) resp. Since we are able to take finite intersection of open T -intervals in the totally ordered set T , we may extend the foregoing to finite chains of 0 < x1 < x2 < . . . < xn , n ≥ 3. Observe that the axioms allow that non-interacting elements, i.e. x such that 0 < x < 1 is the only orderrelation it is involved in, may appear and disappear momentarily. here disappearing means going to 0 under all ϕtt′ , t ≤ t′ , if x ∈ Λt . Very often properties studied are only preserved in some T -interval, in particular this happens often when trying to derive a property of a related system from another one that may be globally defined for T . This leads to an interesting phenomenon, already encoding some aspect of the moment-spaces to be defined later.
2.4
Definition of Observed Truth
A statement in terms of finitely many ingredients of a DNT and depending on parametrization by t ∈ T is said to be an observed truth at t0 ∈ T if there is an open T -interval ]t1 , t2 [ containing t0 , such that the statement holds for parameter values in this interval. It seems that mathematical statements about a DNT turn into “observed truth” when one tries to check them in the commutative shadow, meaning on the negative side that many global (over T ) properties of a DNT cannot be established globally over T in the commutative world ! 8
The noncommutative topologies Λt considered in the sequel will be such that ∨ is commutative and ∨ of arbitrary families exist; in fact one may restrict to so-called “virtual topologies” as introduced in [VO2]; here we do not need to assume axiom (ν) with respect to global covers, we may want to restrict to this case when needed. We refer to the special case mentioned as a DVT.
2.5
Proposition
Let {Λt , ϕtt′ , T } be a DVT and let SL(Λt ) be the commutative shadow of Λt with maps ϕtt′ : SL(Λt ) → SL(Λt′ ), t ≤ t′ in T , just being the restrictions of the ϕtt′ (using same notation). Then the statement that {SL(Λt ), ϕtt′ , T } is a DVT too is an observed truth at every t0 ∈ T . Proof All ϕtt′ map ∧-idempotents to ∧-idempotents, cf. Lemma 2.1., so DNT.1. is obvious. For DNT.2 we have to check that ϕtt′ preserves ∧. on id∧ (Λt ), since ∨. = ∨ now we have nothing to check for ∨. . Look at t0 ∈ T , ϕt0 t : Λt0 → Λt and σ, τ in id∧ (Λt0 ). If σ < τ then ϕt0 t (σ) ≤ ϕt0 t (τ ) and ϕt0 t (σ)∧. ϕt0 t (τ ) = ϕt0 t (σ) = ϕt0 t (σ∧. τ ), interchange the role of σ and τ in case τ < σ. So we may assume σ and τ to be incomparable. Restricting t to a suitable T -interval (DNT 5) we may assume that ϕt0 t (σ) 6= ϕt0 t (τ ). Assume ϕt0 t (σ∧. τ ) < ϕt0 t (σ)∧. ϕt0 t (τ ). If ϕt0 t (σ∧. ϕt0 t (τ ) = ϕt0 t (σ) (a similar argument will hold if σ and τ are interchanged) then ϕt0 t (σ) ≤ ϕt0 t (τ ), hence ϕt0 t (σ) < ϕt0 t (τ ). Using DNT.5 again, taking t close enough to t0 , we obtain σ∧. τ < σ1 < τ such that ϕt0 t (σ∧. τ ) < ϕt0 t (σ1 ) = ϕt0 t (σ) < ϕt0 t (τ ). Passing to [t0 , t] small enough in order to have unambiguity for ϕt0 t (σ), we arrive at σ1 = σ, contradicting the incomparability of σ and τ . Therefore we arrive at strict relations : ϕt0 t (σ∧. τ ) < ϕt0 t (σ)∧. ϕt0 t (τ ) < ϕt0 t (σ), ϕt0 t (τ ). We may moreover assume (DNT.3) that t is close enough to t0 so that there is a z ∈ Λt0 such that σ∧. τ < σ, τ and ϕt0 t (z) = ϕt0 t (σ)∧. ϕt0 t (τ ). If z is not ∧-idempotent, then σ∧. τ < z ∧ z < σ would lead to ϕt0 t (z ∧ z) = ϕt0 t (σ)∧. σt0 t (τ ) because ϕt0 t respects ∧ and the latter is idempotent in Λt ; then ϕt0 t (z) = ϕt0 t (z ∧ z) but the unambiguity guaranteed by the choice of t close enough to t0 (DNT.5) then yields z = z∧z or z ∈ id∧ (Λt0 ). Thus z = σ∧. τ by definition, a contradiction. Consequently, for t in some small enough T -interval containing t0 we have obtained :ϕt0 t (σ∧. τ ) = ϕt0 t (σ)∧. ϕt0 t (τ ), thus DNT.2 is an observed truth. To check DNT.3, take σ < τ in id∧ (Λt1 ), t < t1 such that z1 ∈ id∧ (Λt1 ) exists such that we have ϕtt1 (σ) < z1 < ϕtt1 (τ ). Now by DNT.3. for {Λt , ϕtt′ , T } there is a z ∈ Λt , z < τ , such that ϕtt1 (z) = z1 , and DNT.5 for (Λt , ϕtt′ , T }, used as in foregoing part of the proof, yields ϕtt1 (z) = z1 with z also ∧-idempotent in Λt , for t1 close enough to t. The proof of DNT.4 follows in the same way and DNT.5 is equally obvious because unambiguity in a suitable T -interval allows to pull back idempotency. Therefore all DNT-axioms hold for {SL(Λt ), ϕtt′ , τ ) in a suitable T -interval, hence we have the observed truth statement that {SL(Λt ), ϕtt′ , T } is DNT. Now fix a notion of point i.e. either minimal point or irreducible point as in 9
Section 1. We say that λt ∈ Λt is a temporal point if t ∈]t0 , t1 [ such that for some t′ ∈]t0 , t1 [ there is a point pt′ ∈ Λt′ such that : either t ≤ t′ and ϕtt′ (λt ) = pt′ , or t′ ≤ t and ϕt′ t (pt′ ) = λt ; in the first case we say λt is a future point, in the second case a past point. The system {Λt , ϕtt′ , T } is said to be temporally pointed if for every t ∈ T and λt , ∈ Λt there exists a family of temporal points {pα,t ; α ∈ T } in Λt such that λt is covered by it, i.e. λt = ∨{pα,t , α ∈ A}. Write T P(Λt) for the set of temporal points of Λt , if we write Spec(Λt ) = {pt′ point in Λt′ , pt′ defines a temporal point of Λt } then T P(Λt ) may also be written as T Spec(Λt ) (note T Spec(Λt ) is in Λt but Spec(Λt ) not). We need to build in more “continuity” aspects in the DVT-axioms without using functions or extra assumptions on T e.g. that it should be a group. A temporary poited system {Λt , ϕtt′ , T } is a space continuum if the following conditions hold : SC.1 There is a minimal closed interval Tt containing t in T such that T Spec(Λt ) has support in It . The set of points in Λt′ with t′ ∈ It , representing temporary points in Λt is then called the minimal spectrum for T Spec(Λt ), denoted by Spec(Λt , It ). SC.2 For any open T -interval I such that It ⊂ I there exists an open T -interval It∗ with t ∈ It∗ , such that for t′ ∈ It∗ we have It′ ⊂ I. SC.3 For intervals [t1 , t2 ] and [t3 , t4 ] we write [t1 , t2 ] < [t3 , t4 ] if t1 ≤ t3 and t2 ≤ t4 (similarly for open intervals). If t ≤ t′ in T then It < It′ . This provides an “orientation” of the variation of the minimal spectra ! SC.4 Local Preservation of Directed Sets For given t ≤ t′ in It and any directed set At in Λt , the subset {γt ∈ At , there exists ξt < γt in At such that ϕtt′ (ξt ) < ϕtt′ (γt )} is cofinal in At (defines the same limit [At ]). For t′′ ≤ t in It there is a directed set At′′ in Λt′′ mapped by ϕt′′ t to a cofinal subset of At . A subset J of T is relative open around Q t ∈ T if it is intersection of It and an open T -interval. For x = (. . . , xt , . . .) ∈ t∈T Λt we put sup(x) = {t ∈ T, xt 6= 0}. We say that such an x is topologically accessible if all xt′ t ∈ sup(x), are classical i.e. xt = [χt ] (for some χt ∈ Xt and Λt = C(Xt ). In case we do not consider Λt as coming from some Xt the condition becomes void. An x as before is said to be t-accessible if sup(x) = J is relative open around t and for all t′ ≤ t′′ in J we have ϕt′ t′′ (xt′ ) ≤ xt′′ . When Λt has enough points i.e. if It = {t}, then the points in an open for the point topology would be charactericed by {p, p ≤ [χt ]} = U (χt ) for some χt ∈ Xt . When Λt does not have enough points then we have to modify the definition of point spectrum and point topology correspondingly. If x = (. . . , xt , . . .) is t accessible and pt′ ∈ Spec(Λt , It ) then we say pt′ ∈ x if t′ ∈ J = sup(x) and there exists an open T -interval J1 ⊂ J with t′ ∈ J1 such that for t′′ ∈ J1 we have : if t′ ≤ t′′ then pt′′ = ϕt′ t′′ (pt′ ) ≤ xt′′ , or if t′′ ≤ t′ there is a pt′′ ∈ Λt′′ such that ϕt′′ t′ (pt′′ ) = pt′ , pt′′ ≤ xt′′ , i.e. {pt′′ , t′′ ∈ J1 } is the restriction of a temporal 10
point representing pt′ defined over a bigger T -interval ]t0 , t1 [ containing both t′ and t (note : J1 need not contain t).
2.6
Theorem
The empty set together with the setsQUt (x) = {pt′ , pt′ ∈ x for some t′ ∈ It } ⊂ Spec(Λt , It ), x being t-accessible in t∈T Λt , form a topology on Spec(Λt , It ), called spectral topology at t ∈ T . Proof Consider x 6= y both t-accessible with respective T -intervals J, resp. J ′ contained in It . If pt′ ∈ Ut (x) ∩ Ut (y) then t′ ∈ J ∩ J ′ and for every t1 ∈ J, pt1 ≤ xt1 , for every t2 ∈ J ′ , pt2 ≤ yt2 . Of course the interval J ∩ J ′ is relative open around t. If t′ ≤ t′′ with t′′ ∈ J ∩ J ′ then ot′′ = ϕt′ t′′ (pt′ ) is idempotent in Λt′′ because pt′ is in Λt′ as it is a point. Hence we obtain : pt′′ = pt′′ ∧ pt′′ ≤ xt′′ ∧ yt′′ Obviously for all t′′ ≤ t′′′ in J ∧ J ′ we do have : ϕt′′ t′′′ (xt′′ ∧ yt′′ ) ≤ xt′′′ ∧ yt′′′ . On the other hand, for t′′ ≤ t′ we obtain : ϕt′′ t′ (pt′′ ) = pt′ and therefore pt′ ≤ ϕt′′ t′ (xt′′ ) ≤ xt′ , as well as pt′ ≤ ϕt′′ t′ (yt′′ ) ≤ yt′ . Hence, again by idempotency of pt′ in Λt′ we arrive at pt′ ≤ xt′ ∧ yt′ . By restricting J ∩ J ′ to the interval obtained by allowing only those t′′ ≤ t′ which belong to an (open) unambiguity interval for pt′ we arrive at a relative open around t, say J ′′ ⊂ J ∩ J ′ , containing t′ . Now for pt′′ with t′′ ∈ J ′′ it follows that pt′′ is idempotent because both pt′′ and pt′′ ∧pt′′ map to pt′ via ϕt′′ t′ for t′′ ≤ t′ (other t′′ in J ′′ are no problem). Thus for t′′ in J ′′ we do arrive at pt′′ ≤ xt′′ ∧ yt′′ . Difine w by putting wt′′ = xt′′ ∧ yt′′ for t′′ ∈ J ′′ . Clearly, w is t-accessible and pt′ ∈ Ut (w). Conversely if pt′ ∈ Ut (w) then pt′ ∈ Ut (x) ∩ Ut (y) is clear because J ′′ used in the definition of w is open in J ∩ J ′ . Now we look at a union of Ui,t = Ut (xi ) for i ∈ J and each xi being A-accessible with corresponding relative open interval Ji in It . Define w over the “interval” J = ∪i {Ji , i ∈ J } by putting wt = ∨{xi,t , i ∈ J } for t ∈ J. It is clear that J is relative open around t and for all t1 ≤ t2 in J we have ϕt1 t2 (wt1 ) ≤ wt2 because ϕt,t2 respects arbitrary ∨. Now pt′ ∈ w means that pt′′ ≤ ∨{xi,t′′ , i ∈ J } for t′′ in some relative open containiing t′ , say J1 ⊂ J. We use relative open sets in T because It was closed and there are two situations to consider concerning t′ ∈ It . First if t′ is the lowest element of It then for all t′′ ∈ J1 we have that pt′′ = ϕt′ t′′ (pt′ ) ≤ ϕt′ t′′ (∨{xi,t′ , i ∈ J }) and for all t′ ≤ t1 ≤ t′′ we also obtain : pt1 ≤ ϕt′ t1 (∨{xi,t′ , i ∈ J }) and pt′′ ≤ ϕt1 t′′ (∨{xt,t1 , i ∈ J }). Otherwise, if t′ is not the lowest element of It then we may restrict J1 to be an open interval ]t0 , t′0 [ containing t′ with t0 ∈ J. The same reasoning as in the first case yields for all t′′ ∈]t0 , t′0 [ so that : pt′′ ≤ ϕt0 t′′ (∨{xi,t0 , τ ∈ J }) and for any t′ ≤ t1 ≤ t′′ pt′′ ≤ ϕt1 t′′ (∨{xi,t1 , i ∈ J }) = ∨{ϕt1 t′′ (xi,t1 ), i ∈ J }. Since t′ ∈ J1 we obtain pt′ ≤ ∨{ϕt1 t′ (xi,t1 ), i ∈ J} for all t1 ∈ [t0 , t′ ]. Since pt′ is a point in Λt′ there is an i0 ∈ J such that pt′ ≤ ϕt0 t′ (xi0 ,t0 ) and therefore we have that pt′ ≤ ϕt1 t′ (xi0 ,t1 ) with t1 ∈ [t0 , t′ ], the gain being that 11
i0 does not depend on t1 here ! Now for t′′ ≥ t′ in J1 ∩ Ji0 (note that this is not empty because xi0 is nonzero at t0 because pt′ ≤ ϕt0 t′′ (xi0 ,t0 ) would then make pt′ zero and we do not look at the zero (the empty set) as a point of Λt ) we obtain : (∗)
pt′′ = ϕt′ t′′ (pt′ ) ≤ ϕt′ t′′ (xi0 ,t′ ) ≤ xi0 ,t′′
In the other situation t′′ ≤ t′ in J1 ∩ Ji0 we have ϕt′′ t′ (pt′′ ) = pt′ , ϕt′′ t′ (xi0 ,t′′ ) ≤ xi0 ,t′ . By restricting J1 ∩ Ji0 further so that the t′′ ≤ t′ are only varying in an (open) unambiguity interval for pt′ , say J2 ⊂ J1 ∩ Ji0 , we arrive at one of two cases : either pt′′ = xi0 ,t′′ or else pt′′ 6= xi0 ,t′′ and also pt′ < xi0 ,t′ . In the first case pt′ ∈ xi0 follows because pt1 = ϕt′′ t1 (xi0 ,t′′ ) ≤ xi0 ,t1 for t1 in ]t′′ , 1[∩J2 , the latter interval containing t′ is relative open again. In the second case we may look at pt′ < xi0 ,t′ < 1, hence there exists a zt′′ such that pt′′ < zt′′ < 1 and ϕt′′ t′ (zt′′ ) = xi0 ,t′ . Again we have to distinguish two cases, first ϕt′′ t′ (xi0 t′′ ) = xi0 ,t′ or ϕt′′ t′ (xi0 ,t′′ ) < xi0 ,t′ . In the first case zt′′ and xi0 ,t′′ map to the same element via ϕt′′ t′ , hence up to restricting the interval further such that t′′ stays within an unambiguity interval for xi0 ,t′ , we may conclude zt′′ = xi0 ,t′′ in this case and then pt′′ < xi0 ,t′′ . In the second case we may look at : pt′ ≤ ϕt′′ t′ (xi0 ,t′′ ) < xi0 ,t′ < 1 (where the first inequality stems from (*) above). Again restricting the interval further (but open) we find a zt′′′ in Λt′′ such that xi,t′′ < zt′′′ < 1 such that ϕt′′ t′ (zt′′′ ) = xi0 ,t′ . Since we are dealing with the case pt′′ 6= xi0 ,t′′ and we are in an unambiguity interval for pt′ it follows that pt′ < ϕt′′ t′ (xi0 ,t′′ ). Look at : pt′ < ϕt′′ t′ (xi0 ,t′′ ) < xi0 ,t′ with ϕt′′ t′ (pt′′ ) = pt′ and ϕt′′ t′ (zt′′′ ) = xi0 ,t′ ; by restructing the interval (open) further if necessary we obtain the existence of zt′′′′ such that, pt′′ < z′′′′ < zt′′′ such that ϕt′′ t′ (zt′′′′ ) = ϕt′′ t′ (xi0 ,t′′ ). Finally restricting again the t′′ ≤ t′ to vary in an unambiguity interval for ϕt′′ t′ (xi0 ,t′′ ) it follows that zt′′′′ = xzi0 ,t′′ and hence zt′′′′ ≥ pt′′ yields xi0 ,t′′ ≥ pt′′ for t′′ in a suitable relative open around t containing t′ . This also in the case we arrive at pt′ ∈ xi0 or pt′ ∈ Ut (xi0 ). It follows that Ut (w) = U {Ui,t , i ∈ J } establishing that arbitrary unions of opens are open. By taking Ut (1) we obtain the whole spectrum at t as an open too.
2.7
A Possible Relation to M-Theory ?
In noncommutative topology and derived point topologies the gen-topology appears naturally (and it is a classical i.e. commutative topology (cf. [VO1]). Moreover continuity in the gen-topology also appears naturally in noncommutative geometry of associative algebras but we did not ask the ϕtt′ in the DNTaxioms to be continuous in the gen topology. However one may prove that in general “continuity of the ϕtt′ in the gen-topologies of Λt resp. Λt′ is an observed truth ! Consequently for t′ close enough to t the ϕtt′ is continuous with respect to the gen-topologies (cfr. [VO2]). In the mathematical theory all Λt may be different and there is no reason to aim at Sl(Λt ) nor Spec(Λt , It ) to be invariant under t-variation. From the point
12
of view of Physics one may reason that only one case is important i.e. the case we see as “ reality”. This being the utmost special case it is then not far-fetched to assume that the dynamic noncommutative space has as commutative shadow the abstract mathematical frame we reason in about reality. for example identified with 3 or 4 dimensional space or space-time, and also an observational mathematical frame we measure in that is Spec(Λt , It ) identified to an 11-dimensional space (for M -theory) or any other one fitting physical interpretations in some theory one chooses to believe in. An observed point in Spec(Λt , It ) is then given by a string of elements say pt′ ∈ Λt′ , t′ ∈ J ⊂ It with pt ∈ Λt a temporal point. If pt′ is a point then for all t′ ≤ t′′ , ϕt′ t′′ (pt′ ) is idempotent so appears in the commutative shadow SL(Λt′′ ). Hence an observed point in Spec(Λt , It ) appears as a string in the base space SL(Λt′′ )(t′ ≤ t′′ ), identified with n-dimensioal space but the string may “start after” t when the point was “observed”. On the other hand, the assumption that the system (Λt , ϕtt′ , T } is temporally pointed leads to a decomposition of of every pt′ into temporal points of Λt′ realizing it as an open of Spec[Λt′ , It′ ]. Thus in the spectral space (identified with a certain m-dimensional space say), the observed point appears as a “string” connecting opens i.e. a possibly more dimensional string that can be thought of as a tube. The difference between the dimensions m and n has to be accounted for by the “rank” of T (e.g. if T were a group like Rd+ , d would be the rank) which allowed to create the extra points in Spec(Λt , It ) when compared to (SL(Λt ). Note that even when the ϕtt′ do not necessarily define maps between Spec(Λt , It ) and Spec(Λt′ , It′ ) or between SL(Λt ) and SL(Λt′ ), the given strings at the Λt -level do define sequences of elements or opens in the Spec(Λt , It ) resp. SL(Λt ) that may be viewed as strings, resp. tubes. Two more intriguing observations : i) Identifying Spec(Λt , It ) to one fixed commutative world and SL(Λt ) to another allows strings and tubes to be open or closed. ii) Only temporal points corresponding to future points can be nonidempotent, therefore all noncommutativity is due to future points and uncertainty may be seen as an effect of the possibility that the interval needed to realize the temporal point pt by a point pt′′ ∈ Λt′′ for t ≤ t′′ is larger than the unambiguity interval for pt′′ ! Passing from a commutative frame (SL(Λ)) to noncommutative (dynamical) geometry and phrasing theories and calculations in Spec(Λt , It ) at the price of having to work in higher dimension seems to fit quantum theories. Of course this is at the level of mathematical formalism, for suitable interpretations within physics the physical entity connected to the notion of point in Spec(Λt , It ) should be the smallest possible, i.e. a kind of building block of everything, so that observing it as a point in the moment spaces is acceptable; that these points are mathematically described as strings or higher dimensional strings via the noncommutative geometry is a “Deus ex Machina” pointing at an unsuspected possibility of embedding M -theory in our approach. No further speculation about this here, perhaps specialists in string theory may be interested in investigating further 13
this formal incidence.
3
Moment Presheaves and Sheaves
Continuing the point of view put forward in the short introduction to Section 2, points or more precisely functions defined in a set theoretic spirit, should be replaced by a generalization of “germs of functions” obtained from limit constructions in classical topology terms to noncommutative structures. Thus the notion of point is replaced by an atavar of the notion “stalk” of a pregiven sheaf, more correctly when different (pre)sheaves over a noncommutative space are being considered, say with values in some nice category of objects C, then a “point” is a suitable limit functor on C-objects generalizing the classical construction of localization (functor) at a point. Assuming that a suitable topological space and satisfactory sheaf of “functions” on it have been identified, satisfactory in the sense that it allows to study the desired geometric phenomena one is aiming at, then the notion of point via stalks should be suitable too. For example, prime ideals would be identified via stalks of the structure sheaf of a commutative Noetherian ring without having to check a primeness condition of the corresponding localization functor. More on the definition of noncommutative geometry via (localization) functors can be found in [VO1] where it is introduced as a functor geometry over a noncommutative topologyee also [MVO] and [VOV]. In this section we fix a category C allowing limits and colimits; we might restrict to Abelian or even Grothendieck categories but that is not essential. In fact, the reader who wants to fix ideas on a concrete situation may choose to work in the category of abelian groups. For every t ∈ T, Γt is a presheaf over Λt and for t ≤ t′ in T there is a ′ φtt : Tt → Tt′ , defined by morphisms in C as follows : i) For λt ∈ Λt there is a φtt′ (λt ) : Γt (λt ) → Γt′ (ϕtt′ (λt )) ii) for µt ≤ λt in Λt we have commutative diagrammes in C : Γt (λt )
φtt′ (λt )
/
′
ρtλt ,µt
Γt (µt )
Γt′ (ϕtt′ (λt )) ρtλ
/ φtt′ (µt )
,µ ′ t′ t
Γt′ (ϕtt′ (µt ))
where we have written λt′ , µt′ for ϕtt′ (λt ), resp. ϕtt′ (µt ) and ρtλt′ µt for the restriction morphism of Γt . iii) φtt (λt ) = IΓt (λt ) for all t ∈ T , and for t ≤ t′ and let t′ ≤ t′′ we have φt′ t′′ (φtt′ (Tt (λt ))) = φtt′′ (Tt (λt )) for all λt ∈ Λt . The system {Γt , φtt′ , T } is called a (global) dynamical presheaf over the DNT {Λt , ϕtt′ , T }. 14
Since sheaves on a noncommutative topology do not form a topos it is a problem to define a suitable sheafification i.e. : can a presheaf Γ on Λ be sheafified to a sheaf aΓ on the same Λ via a suitable notion of “stalk”, then allowing interpretations in terms of “points” ? In fact, the axioms of DNT allow to give a solution to the sheafification problem at the price that the sheaf aΓt has to be constructed over Spec(Λt , It ) ! From hereon we let Λ = {Λt , ϕtt′ , T } be a temporally pointed system which is a space continuum. We refer to Yt = Spec(Λt , It ) with its spectral topology as the moment space at t ∈ T . For pt′ ∈ Yt we may calculate (in C): Γt′ ,pt′ = lim Γt′ (xt ) where lim is over −→
−→
xt′ ∈ Λt′ such that pt′ ≤ xt′ , and where x = (. . . , xt , . . .) is t-accessible, in fact we have pt′ ∈ x. In the foregoing we did not demand Λ to derive from a system X and passing from Xt to Λt as a generalized Stone space or pattern topology via C(Xt ),. We preferred not to dwell upon the formal comparison of dynamical theories for the Xt and the Λt . In order to keep trace of a possible original Xt if it was considered in the construction of Λt one may if desired use the following.
3.1
Definition
We say that ut ∈ Λt is classical if ut = [χt ] for χt ∈ Xt . If ut is classical then there is an open interval containing t in T , say L, such that for every t′ ∈ L we have that ut′ is classical, where for t ≤ t′ we have ut′ = ϕtt′ (ut ) and for t′ ≤ t, ut′ is a suitably chosen representative for ut , ϕt′ t (ut′ ) = ut . Restricting further to an unambiguity interval of ut , the representations ut′ for t′ in that interval are unique. Since points of Xt are by definition elements in C(Xt ), the filter in Λt defined by that point has a cofinal directed subset of classical elements. When in {Λt , ϕtt′ , T } we restrict attention to classical x, i.e. every xt classical Λt , then we say that we look at a traditional system.
3.2
Lemma
For a traditional space continuum with dynamical presheaf {Γt , φtt′ , T }, the stalk for pt′ ∈ Yt of Γt′ is exactly Γt′ ,pt′ as defined above. Proof In calculating lim{Γt′ (ut′ ), pt′ ≤ ut′ } we may restrict to classical ut′ −→ in Λt′ . It now suffices to establish the existence of a t-accessible y such that pt′ ∈ y and yt′ ≤ ut′ . From pt′ ∈ Ut (x) we obtain (. . . , xt′ , . . .) with a relative open T -interval J, t′ ∈ J, such that pt′′ ≤ xt′′ for every t′′ ∈ J. Since ut′ and xt′ are classical, so is ut′ ∧ xt′ and moreover pt′ ≤ ut′ ∧ xt′ because pt′ is a point in Λt′ (hence idempotent !). Let J1 be an open T -interval containing t′ such that ut′ ∧ xt′ has a representative ut′′ in Λt′′ such that ϕt′′ t′ = ut′ ∧ xt′ . Since xt′ 6= ut′ ∧ xt′ may be assumed (otherwise put y = x) we arrive at pt′′ ≤ ut′′ < xt′′ . Using the intersection of J1 and the interval iaround t′ allowing to select classical ut′′ , call this interval J2 , we put yt′′ = ut′′ for t′′ ≤ t′ in J2 and yt1 = xt1 for t′ < t1 in J. Then y is t-accesible with respect to the
15
relative open T -interval around t just defined : we have yt′ ≤ ut′ and pt′ ∈ y. Consequently : lim Γt′ (ut′ ) = lim Γt′ (xt′ ). −→
−→
p ′ ≤u ′ t t
p ′ ∈x t
In the sequel we assume objects in C are at least sets but let us restrict to abelian groups. Again let {Γt , φtt′ , T } be a dynamical presheaf over a traditional space continuum. On Yt we define a presheaf, with ` respect to the spectral topology, by taking for P(Ut (x)) the abelian group in t′ ∈It Γt′ (xt′ )formed by strings over sup(x) = {t′ ∈ It , xt′ 6= 0}. i.e. {γt′ , t′ ∈ sup(x), φt′′ t′ (γt′′ ) = γt′ for t′′ ≤ t′ in sup(x)}. Let us write x < y if xt′ ≤ yt′ for all t′ in It , in particular x < y means sup(x) ⊂ sup(y). In sheaf theory one usually omits the empty set, so here the o ∈ Λt at every t, and it makes sense to do that here as well. However one may define at every t′ ∈ T , Γt′ (0) = lim{Γt′ (xt′ ), xt′ classical in −→
Λt′ } and all statements made in the sequel will remain consistent. ′ If x < y then we have restriction morphisms ρtyt′ ,xt′ , : Γt′ (yt′ ) → Γt′ (xt′ ). Commutativity of the diagrams in the beginning of the section yield corresponding morphisms on the strings over the respective supports : ρy,x : P(Ut (y)) → P(Ut (x)). For a point pt′ we let η(pt′ ) be the set of Ut (x) such that we have pt′ ∈ Ut (x) i.e. pt′ ∈ x; in particular t′ ∈ Jx where Jx is the relative open around t in the definition of x and consequently : t′ ∈ ∩{sup(x), η(pt′ ) contains Ut (x)}. For the dynamical sheaf theory we may want to impose coherence conditions on the system assuming some relations between Γt′′ and Γt′ if t′ and t′′ are close enough in T . We shall restrict here to only one extra assumption, in some sense dual to the unambiguity interval assumption for the underlying DNT.
3.3
Definition
The dynamical presheaf {Γt , φtt′ , T } on a traditional space continuum is locally temporaly flabby at t ∈ T if for t-accessible x such that pt′ ∈ x and st′ ∈ Γt′ (xt′ ) there exists a t-accessible y < x with pt′ ∈ y and a string s ∈ P(Ut (y)) such ′ that st′ = ρtxt′ ,yt′ (st′ ).
3.4
Theorem
To a dynamical presheaf on a traditional space continuum there corresponds for every t ∈ T a presheaf Pt on the moment space Spec(Λt , It ) with its spectral topology given by the Ut (x) for t accessible x. In case all Γt′ , t′ ∈ It , are separated presheaves then Pt is separated too. The sheafification aPt of Pt on the moment space Spec(Λt , It ) = Yt is called the moment sheaf of spectral sheaf at t ∈ T . In case the dynamical presheaf is locally temporally flabby (LTF) then for any point pt′ ∈ Yt the stalk Pt,pt′ may be identified with Γt′ ,pt′ . Proof At every t ∈ T , Pt is the spectral presheaf constructed on Spec(Λt , It ) with its spectral topology. Now suppose all Γt are separated presheaves and look at a finite cover Ut (x) = Ut (x1 ) ∪ . . . ∪ Ut (xn ) and a γ ∈ Γt (Ut (x)) such that for i = 1, . . . , n, ρx.xi (γ) = 0. We have seen before that the union Ut (x1 ) ∪
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. . . ∪ Ut (xn ) corresponds to the t accessible element x1 ∧ . . . ∧ xn obtained as the string over sup(x1 ) ∧ . . . ∧ sup(xn ) given by the x1,t′ ∪ . . . ∪ xn.t′ in Λt′ . For all t′ ∈ sup(x) we obtain, in view of the compatibility diagrams for ′ restrictions and φt′ t′′ , t′ ≤ tn : ρtxt′ ,xi,t′ (γt′ ) = 0, for i = 1, . . . , n. The assumed separatedness of Γt′ , for all t′ then leads to γt′ = 0 for all t′ ∈ sup(x) and therefore γ = 0 as a string over sup(x). Consequently Pt is separated, for all t ∈ T . In order to calculate the stalk at pt′ ∈ Spec(Λt , It ) for Pt we have to calculate : lim Γt (Ut (x)) = Et′ . −→
p ′ ∈x t
Starting with pt′ ∈ x for some t-accessible x we have a representative γx ∈ Γt (Ut (x)) being a string over sup(x) and the latter containing a relative open J(x) around t containing t′ . So an element et′ in Et′ may be viewed as given by a direct family {γx , pt′ ∈ x, ρx,y (γx ) = γy for y < x}. At t′ , which is in sup(x) for all x appearing in the forgoing family (as Ut (x) varies over η(pt′ )), we obtain ′ {(γx )t′ , pt′ ≤ xt′ , ρtxt′ ,yt′ ((γx )t′ ) = (γy )t′ which defines an element of Γt′ ,pt′ , say et′ . We have a well-defined map πt′ : Et′ → Γt′ ,pt′ , et′ 7→ et′ . Without further assumptions we therefore arrive at a sheaf aPt with stalk Et′ at pt′ and a presheaf map Pt → aPt which is “injective” in case all Γt′′ are separated. Now we have to make use of the locally temporally flabbyness (LTF). Look at a germ st′ ∈ (Γt′ )pt′ . In view of Lemma 3.2. there exists a t-accessible x such that st′ ∈ Γt′ ((xt′ ) with pt′ ∈ x, in particular pt′ ≤ xt′ . The LTF-condition allows to select a t-accessible y < x with pt′ ∈ y together ~ ∈ P(Ut (y)) such that s~t′ (y) = ρt′ with a string, s(y) xt′ ,yt′ (st′ ). The element et′ in Et′ defined by the directed family obtained by taking restrictions of s~t′ (y) has et′ exactly st′ (note that t′ supports all the restrictions of s~t′ (y) because y varies in η(pt′ )). Thus πt′ : Et′ → Γt′ ,pt′ is epimorphic. If et′ and e′t′ have the same image under πt′ then there is a t-accessible y such that et′ − e′t′ is represented by the zero-string over sup(y); in fact this follows by taking st′ = 0 in the foregoing. Leading to a t-accessible y as above that may be restricted to a t-accessible y ′ defined by taking for sup(y ′ ) the relative open J containing t′ in the support of y where s~t′ (y) = 0. Therefore, πt′ is also injective. Can one avoid a condition like LT F in the foregoing theorem ? It seems that the idea of “germ” appearing in the notion of stalks spatially needs an extension in the temporal direction, so probably some condition close to LT F is really necessary here.
4
Some Remarks on Spectral families and Observables
Let Γ be any totally ordered abelian group. On a noncommutative topology Λ we define a Γ-filtration by a family {λα , α ∈ Γ} such that for α ≤ β in Γ, λα ≤ λβ in Λ and ∨{λα , α ∈ Γ} = 1. A Γ-filtration is said to be separated if from γ = inf{γα , α ∈ A} in Γ it follows that λγ = ∧{λα , α ∈ A} and 0 = ∧{λγ , γ ∈ Γ}. A Γ-spectral family in Λ is just a separated Γ-filtration, it
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may be seen as a map F : Γ → Λ, γ 7→ λγ , where F is a poset map satisfying the separatedness condition. Note that by definition the order in ∧{λγ , γ ∈ Γ} does not matter but λγα need not be idempotent in Λ. Taking Γ = R+ and Λ = L(H) the lattice of a Hilbert space H, we recover the usual notion of a spectral family. We say that a Γ-spectral family on Λ is idempotent if λγ ∈ id∧ (Λ) for every γ ∈ Γ. Observation If Γ is indiscrete, i.e. for all γ ∈ Γ, γ = inf{τ, γ < τ } (example Γ = Rn+ ), then every Γ-spectral family is idempotent.
4.1
Proposition
i) Let us consider a Γ-spectral family on Λ, then for γ, τ ∈ Γ : λγ ∧λτ = λτ ∧ λγ = λδ , where δ = min{τ, γ}. ii) If the Γ-spectral family is idempotent then for γ, τ ∈ Γ, λγ ∧ λτ = λτ ∧ λγ and the Γ-spectral family on Γ is in fact a Γ-spectral family of the commutative shadow SL(Λ). Proof Easy enough. A filtration F on a noncommutative Λ is said to be right bounded if λγ = 1 for some γ ∈ Γ, F is left bounded if λδ = 0 for some δ ∈ Γ. For a right bounded Γ-filtration F : Γ → Λ we may define for every µ ∈ Λ the induced filtration F |µ : Γ → Λ(µ) where we use µ = 1∧(µ) , Λ(µ) = {λ ∈ Λ, λ ≤ µ}. Note that F |µ need not be separated whenever F is, indeed if δ = inf{δα , α ∈ A} in Γ then λδ = ∧{λδα , α ∈ A} in Γ then λδ = ∧{λδα , α ∈ A} but µ ∧ λδ , and ∧{µ ∧ λα , α ∈ A} need not be lequal in general.
4.2
Proposition
If F defines a right bounded Γ-spectral family on Λ then F |µ is a spectral family of Λ(µ) in each of the following cases : a. µ ∈ id∧ (Λ) and µ commutes with all λα , α ∈ Γ. b. µ ∧ λα is idempotent for each α ∈ Γ. Proof Easy and straightforward. An element µ with property a. as above is called an F -centralizer of Λ.
4.3
Corollary
In case Λ is a lattice then for every µ ∈ Λ a right bounded Γ-spectral family of Λ induces a right bounded spectral family on Λ(µ). Let F be a Γ-spectral family on a noncommutative Λ. To λ ∈ Λ associate σ(λ) ∈ Γ ∪ {∞} where σ(λ) = inf{γ, λ ≤ λγ } and we agree to write infφ = ∞.
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The map σ : Λ → Γ ∪ {∞} is a generalization of the principal symbol map in the theory of filtered rings and their associated graded rings. We refer to σ as the observable function of F . Clearly : σ(λ ∧ µ) ≤ min{σ(λ), σ(µ)} σ(λ ∨ µ) ≤ max{σ(λ), σ(µ)} The domain of σ is ∪{[0, λγ ], γ ∈ Γ} and observe that ∨{λγ , γ ∈ Γ} = 1 does not imply that D(σ) = Λ (can even be checked for Γ = R+ , Λ = L(H)). If F : T → L(H) is a Γ-spectral family and V ⊂ H a linear subspace, then we may define γ∨ ∈ Γ, γV = inf{γ ∈ Γ, ⊂ L(H)γ }, again putting infφ = ∞. The map ρ : L(H) → Γ ∪ {∞}, U 7→ ρ(U ) = γU is well-defined. One easily verifies for ∪ and ∨ in H : ρ(U + V ) ≤ max{ρ(U ), ρ(V )} ρ(U ∩ V ) ≤ min{ρ(U ), ρ(V )} The function ρ defines a ρ defined on H by putting ρ(x) = ρ(Cx). We denote ρ again by ρ and call it the pseudo-place of the Γ-spectral family. Then any Γ-spectral family defines a function on the projective Hilbert space P(H) described on the lines in H by ρ : P(H) → Γ, Cv 7→ ρ(Cv), where we wrote Cv for Cv as an object in P(H). The pseudo-place aspect of ρ translates to ρ in the following sense : Cw ⊂ Cv + Cu we have ρ(Cw) ≤ max{ρ(Cv), ρ(Cu)}. A linear subspae U ⊂ H such that P(U ) ⊂ ρ−1 (] − ∞, γ]) allows for u 6= 0 in U : ρ(Cu) ≤ γ i.e. u ∈ L(H)γ . Hence, the largest U in H such that P(U ) is in ρ−1 (] − ∞, γ]) is exactly L(H)γ ; this means that the filtration F may be reconstructed from the knowledge of ρ. One easily recovers the classical result that maximal abelian Von Neumann regular subalgebras of L(H) correspond bijectively to maximal distributive lattices in L(H). Since any Γ-spectral family is a directed set in Λ it defines an element of C(Λ) which we call a Γ-point. The set of Γ-points of Λ is denoted [Γ] ⊂ C(Λ). We may for example think of [R] ⊂ C(L(H)) as being identified via the Riemann-Stieltjens integral to the set of self-adjoint operators on H. Let σ : Λ → Γ ∪ {∞} be the observable functions of a Γ-spectral family on Λ defined by F : Γ → Λ. Put F : C(Λ) → Γ ∪ {∞},[A] 7→ inf{γ ∈ Γ, λγ ∈ A}, A the filter of A. Then σ b is the observable corresponding to the Γ-filtration on C(Λ) defined by [A]γ , where for γ ∈ Γ,[A]γ is the class of the smallest filter containing λγ i.e. the filter {µ ∈ Λ, λγ ≤ µ}. This is clearly a Γ-spectral family because in fact [A]γ < [λγ ]. We define [Γ] ∩ Sp(Λ) = Γ−Sp(Λ), [Γ] ∩ QSp(Λ)[Γ] ∩ QSp(Λ) = Γ − QSp(Λ) and similarly with p replaced by P when Id∧ (C[Λ]) is considered instead of id∧ (C(Λ− )) (see section 1). In view of Proposition 4.1.i. a Γ-spectral family is contained in a sublattice (that is with commutative ∧) of the noncommutative Λ, in fact {λγ , γ ∈ Γ} is such a sublattice. If Ab(Λ) is the set of maximal commutative sublattices of Λ then every Γ-spectral family in Λ is a Γ-spectral family in some B ∈ Ab(Λ) (B 19
refers to Boulean sector in case Γ = R+ , Λ = L(H)). The above remarks may be seen as a generalization of the result concerning maximal commutative Von Neumann regular subalgebras in L(H) quoted above. Γ-spectral families may be defined on the moment spaces Spec(Λt , Tt ) in exactly the way described above as filtrations {Ut (xγ ), γ ∈ Γ}, where each xγ is t-accessible, defining a separated Γ-filtration. For t′′ ∈ It we may look at Vt (xγ ) = {pt′′ , pt′′ ∈ Ut (xγ )}, agaiin pt′′ = ϕt′ t′′ (pt′ ) or ϕt′ t′′ (pt′ ) = pt′′ depending whether t′ ≤ t′′ or t′′ ≤ t′ . The family {Vt (xγ ), γ ∈ Γ} need not (!) be a Γ-spectral family at t′′ ∈ T . A stronger notion of dynamical spectral family may be obtained by demanding the existence of stringwise spectral families in a relative open T -interval J around t. Then indeed at t′′ ∈ J ⊂ I(t) such a stringwise Γ-spectral family induces a γ-spectral famly in Λt′′ but not immediately on Spec(Λt′′ , Itn ) unless a more stringent relation is put on Itn and its comparison with respect to It . We just point out the interesting problems arising with respect to observables when passing to moment spaces but this is work in progress.
References [VO1 ] F. Van Oystaeyen, Algebraic Geometry for Associative Algebras, M. Dekker, Math. Monographs, Vol. 232, New York, 2000. [Co ] A. Connes, C ∗ -Alge`ebres et G´eom´etries Differentielle, C. R. Acad. Sc. Paris, 290, 1980, 599-604. [CoD A. Connes, M. Dubois-Violette, Noncommutative Finite Dimensional Manifolds I, Spherical Maniforls and related Examples, Notes IHES, MO1, 32, 2001. [A.T.V. ] M. Artin, J. Tate, M. Van den Bergh, Modules over Regular Algebras of Dimension 3, Invent. Nath. 106, 1991, 335-388. [VdB ] M. Van den Bergh, Blowing up of Noncommutative Smooth Surfaces, Mem. Amer. Math. Soc., 154, 2001, no. 734. [Ko ] M. Kontsevich, Deformation, Quantization of Poissin Manifolds, q-alg, 9709040, 1997. [VO.2 ] F. Van Oystaeyen, Virtual Topology amd Functor Geometry, lecture Notes UA, Submitted. [MVO ] D. Murdoch, F. Van Oystaeyen, Noncommutative Localization and Sheaves,. J. of Algebra, 35, 1975, 500-525. [S ] J.-P. Serre, Faisceaux Alg´ebriques Coh´erents, Ann. Math. 61, 1955, 197-278. [VOV ] F. Van Oystaeyen, A. Verschoren, Noncommutative Algebraoc Geometry, LNM 887, Springer Verlag, berlin, 1981.
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